Current wireless communication systems use spectral efficiency modulation of RF carriers (e.g. W-CDMA, LTE, WiMAX, and MSR). In these systems, radio downlink signal is characterized to have high peak-to-average ratio (PAR). To amplify such signal, it is required to use high-linearity power amplifier (PA), which results in low PA efficiency and high cost. CFR algorithms are widely used to reduce PAR. CFR with proper threshold can significantly reduce PAR while having acceptable error vector magnitude (EVM) degradation, leading to improved efficiency and reduced cost.
Different CFR thresholds result in different EVM performance and PAR values. Higher threshold means worse (higher) PAR but better EVM, while lower threshold means better (lower) PAR and worse EVM. EVM is limited by modulation accuracy requirement and PAR is limited by average power level and PA peak power capability. CFR thresholds and average power level should be properly selected so that good EVM can be achieved while peak power value, which can be calculated as the sum of PAR and average power level, do not exceed PA peak power capability.
PAR and EVM is decided not only by CFR threshold, but also by carrier configurations, for example, carrier radio standards, carrier numbers, modulation types, carrier frequency spacing and power allocation among carriers.
CFR thresholds and average power levels are normally predefined for different carrier configurations. This is all right when radio units support limited carrier configurations. But now, modern radio units are expected to support complex carrier configurations including multi-standards radio, multi carrier numbers, different modulation types, wide instantaneous band width (IBW), and flexible power allocation among different carriers. See FIGS. 1 and 2. There are thousands of different carrier combinations, so it is impossible to set or predefine CFR threshold and average power level for every possible carrier configuration.
One possible solution is to divide all carrier configurations into several groups and use a general rule for each group. In order to have all carrier configurations in the group to meet EVM and PA peak power capability requirement, the worst carrier configuration defines CFR threshold and average power level settings. Drawback is that the settings are not optimized for each carrier configuration. This results in hardware capability waste and it is a time consuming task to find general rules for different groups.
And since the same clipping thresholds are used for all radio units, radio unit with the lowest PA peak power capability is used to define target PAR for clipping. This results in waste of hardware performance.
The document CN101969423A describes a method for setting CFR threshold based on the output of PA. The disclosed embodiments improve the efficiency and precise of PA. However, this document focuses on TD-SCDMA and LTE communication systems only, and it is questionable as to whether it is applicable to other systems. Besides, the disclosed solutions can not obtain optimized EVM.